1. Field of the Invention
The invention is related to the general field of displacement detectors and position encoders and, in particular, to the field of optical instruments using gratings for detecting, measuring and encoding the position of an object or light beam.
2. Prior Art
A variety of optical displacement detectors and position encoders, using gratings in one form or another, are well known in the art. The gratings used in these devices may take the form of bar charts, as disclosed by C. E. Adler, U.S. Pat. No. 2,938,126 -- "Indicator Scanning Device" or a series of parallel apertures in an otherwise opaque mask, as disclosed by F. Hock, U.S. Pat. No. 3,493,775 -- "Optical Scanning Means for Use in Photoelectric Positioning Determining Apparatus". In the simplest forms of these instruments either the image of the grating is scanned across a slit aperture or a slit image scanned across a grating. These systems generally embody a photo detector detecting a transmitted image. The more sophisticated systems use multiple gratings, such as the system disclosed by B. J. Kusch et al., U.S. Pat. No. 3,360,660 -- "Position and Rate Readout Systems with Dual Phase Displaced Gratings", in which a slit image is simultaneously scanned across two parallel gratings disposed with their grating patterns out of phase. Other systems, such as disclosed by S. Albarda, U.S. Pat. No. 3,552,861, used two gratings wherein the pattern of the first grating is imaged on the second grating. Alternatively, as disclosed by Y. Hayamizu in U.S. Pat. No. 3,628,870, optical systems may be employed to reimage the grating pattern back on itself to produce a moire fringe pattern when the grating is moved.
The above disclosed systems, however, necessarily embody relatively coarse grating patterns and use well known electronic means for increasing the resolution by measuring a phase relationship between a moving and a stationary grating. A coarse grating is defined in the context of this disclosure as one in which the separation of the elements and the interspacing are sufficiently large so that effects of Fraunhofer diffraction is negligible. Coarse gratings are contrasted to diffraction type gratings in which the elements and their interspacings are relatively small and Fraunhofer diffraction is prevalent. The use of diffraction type gratings for position determination is very desirable because of their inherently high resolution, however, the resultant Fraunhofer diffraction effectively degenerates the inherent increased spatial resolution. One solution to this problem is disclosed by F. Hock, U.S. Pat. No. 3,482,107 which uses two phase type diffraction gratings. The diffraction pattern of the first grating is imaged on the second grating so that the interspacing of the diffraction orders corresponds to the grating constant. The dispersion by the second grating is dependent upon the position of the diffraction order images with respect to the dispersion centers of the second grating. Therefore, a small movement of either grating can be measured by detecting the change in the dispersion pattern.
The problems associated with the above types of position measuring devices is their extreme sensitivity with respect to the orientation and location of the two gratings with respect to each other. This condition is aggravated by the fact that the system requires one of the gratings to move with the object whose movement is being detected. This factor places extreme restrictions on both design criteria and possible applications.
The disclosed optical device uses a grating in a manner different from that discussed above and overcomes the attendant problems associated with the detectors of the prior art discussed above.